Treating Liquefied Natural Gas

ABSTRACT

Method of treating liquefied natural gas ( 1 ) to obtain a liquid stream ( 21 ) having a reduced content of components having low boiling points comprising expanding ( 3 ) the liquefied gas to expand to obtain expanded two-phase fluid; introducing the two-phase fluid into a column ( 10 ) below a gas-liquid contacting section ( 14 ); withdrawing from the bottom ( 16 ) a liquid-stream ( 17 ) having a reduced content of components having low boiling points; withdrawing from the top ( 23 ) of the column ( 10 ) a gaseous stream ( 25 ) enriched in components having low boiling points; heating the gaseous stream in a heat exchanger ( 27 ); compressing ( 30 ) the stream to fuel gas pressure to obtain fuel gas ( 33 ); separating a recycle stream ( 34   a ) from the fuel gas; at least partly condensing ( 27 ) the recycle stream to obtain a reflux stream ( 34   b ); and introducing the reflux stream ( 34   b ) into the column ( 10 ) above the contacting section ( 14 ).

The present invention relates to treating liquefied natural gas, and inparticular treating liquefied natural gas that contains componentshaving boiling-points lower than methane. An example of such a componentis nitrogen. In the specification and in the claims the expressions ‘lowboiling point components’ and ‘components having low boiling points’will be used to refer to components having boiling points lower thanmethane. The treatment is directed to removing low boiling pointcomponents from the liquefied natural gas in order to obtain a liquefiednatural gas having a reduced content of components having low boilingpoints. The improved method can be applied in two ways: (1) to treat thesame amount of liquefied natural gas as in a conventional method, or (2)to treat a larger amount of liquefied natural gas as in a conventionalmethod. When applied in the first way, the content of low boiling pointcomponents in the liquefied gas treated with the method of the presentinvention is lower than that in liquefied gas treated with aconventional method. When applied in the second way, the content of lowboiling point components is maintained and the amount of liquefied gasis increased.

U.S. Pat. No. 6,199,403 discloses a method to remove a high-volatilitycomponent such as nitrogen from a feed stream rich in methane. Accordingto U.S. Pat. No. 6,199,403 the expanded liquefied natural gas streamenters a separation column at an intermediate level, i.e. not below agas-liquid contacting section.

U.S. Pat. No. 5,421,165 relates to a process for denitrogenation of afeedstock of a liquefied mixture of hydrocarbons. To this end U.S. Pat.No. 5,421,165 suggests a relatively complicated process using adenitrogenation column comprising a plurality of theoreticalfractionation stages.

Another relatively complicated process has been described inInternational patent application publication No. WO 02/50483. WO02/50483 discloses several methods of removing components having lowboiling points from liquefied natural gas. According to WO 02/50483 aliquid product stream having a reduced content of components having lowboiling points is obtained.

A problem of the above processes described in WO 02/50483 is that theliquid product stream contains an undesirable high content of componentshaving low boiling points.

It is an object of the present invention to minimize the above problem.

It is a further object of the present invention to provide analternative process.

It is an even further object of the present invention to provide asimplified process to reduce the amount of components having low boilingpoints in a liquefied natural gas stream.

One or more of the above or other objects are achieved according to thepresent invention by providing a method of treating liquefied naturalgas supplied at liquefaction pressure containing components having lowboiling points to obtain a liquid product stream having a reducedcontent of components having low boiling points, which method comprisesthe steps of:

-   (a) allowing the liquefied gas to expand to separation pressure to    obtain an expanded two-phase fluid;-   (b) introducing the expanded two-phase fluid into a column below a    gas-liquid contacting section arranged in the column;-   (c) collecting in the bottom of the column liquid from the two-phase    fluid and withdrawing from the bottom of the column a liquid stream    having a reduced content of components having low boiling points;    introducing the liquid stream into a flash vessel at a low pressure;    removing a second gaseous stream from the top of the flash vessel;    and removing from the bottom of the flash vessel a liquid stream to    obtain the liquid product stream;-   (d) allowing vapor from the two-phase fluid to flow through the    contacting section;-   (e) withdrawing from the top of the column a gaseous stream that is    enriched in components having low boiling points;-   (f) heating the gaseous stream obtained in step (e) in a heat    exchanger to obtain a heated gaseous stream;-   (g) compressing the heated gaseous stream obtained in step (f) to    fuel gas pressure to obtain fuel gas;-   (h) separating a recycle stream from the fuel gas obtained in step    (g);-   (i) at least partly condensing the recycle stream obtained in    step (h) to obtain a reflux stream; and-   (j) introducing the reflux stream obtained in step (i) at separation    pressure into the column above the contacting section.

Applicants have found that the liquid product stream obtained accordingto the present invention contains a smaller content of components havinglow boiling points than one would expect.

An important advantage of the method according to the present inventionis that it can be suitably used for large liquefaction plants.

The present invention will now be illustrated by way of example in moredetail with reference to the non-limiting accompanying drawings,wherein:

FIG. 1 shows schematically a process flow scheme illustrating a part ofan embodiment of the method of the present invention (not including aflash vessel as required according to the present invention);

FIG. 2 shows schematically an alternative of the process of FIG. 1;

FIG. 3 shows schematically a process flow scheme of a fully elaboratedembodiment of the method of the present invention, including a flashvessel;

FIG. 4 shows schematically an alternative of the process of FIG. 3;

FIG. 5 shows schematically and not to scale an alternative to part V ofthe process flow scheme of FIG. 4; and

FIG. 6 shows the process according to FIG. 4 having two contactingzones.

Reference is made to FIG. 1. Liquefied natural gas containing componentshaving low boiling points is supplied at liquefaction pressure throughconduit 1 to an expansion device in the form of expansion engine 3 andJoule-Thompson valve 5 in the discharge conduit 6 of expansion engine 3.In the expansion device, the liquefied gas is allowed to expand toseparation pressure, and an expanded two-phase fluid is obtained. Theliquefaction pressure is suitably in the range of from 3 to 8.5 MPa andthe separation pressure is suitably in the range of from 0.1 to 0.5 MPa.

The expanded two-phase fluid is passed through conduit 9 to a column 10.The expanded two-phase fluid is introduced into the column 10 atseparation pressure via a suitable inlet device, such as vane inletdevice 12. The vane inlet device, also known as schoepentoeter, allowsefficient separation of gas and liquid.

The column 10 is provided with a gas-liquid contacting section 14. Thecontacting section 14 may comprise any suitable means for contacting agas and a liquid, such as trays and packings. Preferably, the contactingsection 14 consists of between two and eight horizontal contacting trays15. The expanded two-phase fluid is introduced into the column 10 belowthe gas-liquid contacting section 14. The person skilled in the art willreadily understand that the column may comprise two or more contactingsections 14.

In the bottom 16 of the column 10 liquid from the two-phase fluid iscollected, and a liquid stream having a reduced content of componentshaving low boiling points is removed from the bottom 16 through conduit17 and pumped by pump 18 to a storage tank 20. From the storage tank 20a liquid product stream is removed through conduit 21 and a gaseousstream through conduit 22. The gaseous stream is also known as boil-offgas.

Vapor from the two-phase fluid to flow through the contacting section14. From the top 23 of the column 10 a gaseous stream that is enrichedin components having low boiling points is removed through conduit 25.The gaseous stream is heated in a heat exchanger 27 to obtain a heatedgaseous stream that is passed through conduit 28 to a compressor 30. Incompressor 30 the heated gaseous stream is compressed to fuel gaspressure to obtain fuel gas. The fuel gas is removed through conduit 31and cooled in heat exchanger 32 to remove the heat of compression. Thefuel gas is passed away through conduit 33. The fuel gas pressure is inthe range of from 1 to 3.5 MPa.

A recycle stream from the fuel gas and supplied to the heat exchanger 27through conduit 34 a. In the heat exchanger 27 the recycle stream is atleast partly condensed to obtain a reflux stream, which is passed to thecolumn 10 through the conduit 34 b provided with Joule-Thompson valve37. The reflux stream is introduced at separation pressure into thecolumn 10 via inlet device, such as vane inlet device 39 above thecontacting section 14.

Table 1 summarizes the result of a hypothetical example, wherein themethod of FIG. 1 is compared to a base case. In the base case therecycle stream and the feed are introduced into the column at the samelevel, so that the liquid phases of the two streams are mixed beforeintroduction thereof in the column and the column has no contactingsection. It was found that the liquid stream withdrawn through conduit17 for the base case contains more nitrogen than the same stream for thepresent invention. TABLE 1 Summary of hypothetical example with theembodiment of FIG. 1. Embodiment of Base case Number of trays in 3 —contacting section Flow rate feed 190.86 kg/s 190.86 kg/s throughconduit 9 Temperature of −145° C. −145° C. feed introduced through inletdevice 12 Nitrogen content 3.05 mol % 3.05 mol % in feed Recycle flowrate 26 kg/s 26 kg/s Temperature of −165.6° C. −165.2° C. recycleintroduced through inlet device 39 Nitrogen content Vapour phase Totalrecycle of recycle stream contains 33 mol % stream contains Liquid phase22 mol % contains 1.7 mol % Flow rate of 169.25 kg/s 169.19 kg/s productin conduit 21 Nitrogen content 0.65 mol % 0.82 mol % of product inconduit 21 Flow rate of fuel 20.51 kg/s 20.59 kg/s gas in conduit 33Nitrogen content 24 mol % 22 mol % of fuel gas Power required for 30.8MW 31.2 MW compressor 30

Table 1 shows that a lower nitrogen content in the product stream isobtained with the method of the present invention.

In an alternative embodiment the recycle stream separated from the fuelgas is additionally compressed in an auxiliary compressor to an elevatedpressure before it is at least partly condensed in heat exchanger 27.The high-pressure recycle stream can be used in several ways, which willbe discussed with reference to FIG. 2. The parts that were alreadydiscussed with reference to FIG. 1 have got the same reference numerals.

The auxiliary compressor included in conduit 34 a is referred to withreference numeral 35. The auxiliary compressor 35 can be provided with acooler (not shown) to remove the heat of compression for the compressedrecycle stream. The compressed recycle stream is at least partlycondensed by cooling it in heat exchanger 27. Part of the cold that isneeded is provided by the gaseous stream that is enriched in componentshaving low boiling points that is passed through conduit 25. Theremainder is provided by the recycle stream. Cold from the recyclestream can be obtained by expanding a part of the recycle stream to anintermediate pressure in Joule-Thompson valve 38, using the expandedfluid to cool the recycle stream in conduit 34 a and supplying theexpanded fluid through conduit 38 a to the compressor 30. Theintermediate pressure to which the part of the recycle stream isexpanded is in the range of from the suction pressure to the dischargepressure of the compressor 30 (ends of the range included). The stage atwhich the expanded recycle stream enters the compressor 30 is soselected that the pressure of the expanded recycle stream matches thepressure of the fluid in the compressor 30 in that stage.

The remainder of the recycle stream is expanded by the Joule-Thompsonvalve 37 and introduced as reflux in the column 10 as discussed withreference to FIG. 1.

An advantage of the embodiment discussed with reference to FIG. 2 isthat the recycle stream is expanded from a larger pressure and thuscooled to a lower temperature. This allows a warmer feed stream, forexample a feed stream at −142° C., compared to a feed stream temperatureof −145° C. (in the above example). Thus the temperature of theliquefied gas from the main cryogenic heat exchanger can be higher andtherefore, for the same amount of energy, more gas can be liquefied.

The elevated pressure of the fluid discharged from the auxiliarycompressor 35 is so selected that the costs of the power required todrive the auxiliary compressor 35 are less than the value of theincreased amount of gas that is liquefied.

In the above we discussed an embodiment in which the expansion is donein the expansion valves 37 and 38. However, it will be understood thatthe expansion of the recycle stream can be done in two stages, at firstin an expansion device, such as expander 36 and subsequently in theJoule-Thompson valves 37 and 38.

Instead of supplying the expanded fluid through conduit 38 a to thecompressor 30, the expanded fluid can be supplied to an inlet (notshown) of the compressor 35.

In the embodiments discussed with reference to FIGS. 1 and 2, the liquidfrom the two-phase fluid is collected in the bottom 16 of the column 10,and from the bottom 16 a liquid stream 17 is withdrawn having a reducedcontent of components having low boiling points to obtain the liquidproduct stream. In an alternative embodiment of the invention, this stepcomprises collecting in the bottom of the column liquid from thetwo-phase fluid and withdrawing from the bottom of the column a liquidstream having a reduced content of components having low boiling points;introducing the liquid stream into a flash vessel at a low pressure;removing a second gaseous stream from the top of the flash vessel; andremoving from the bottom of the flash vessel a liquid stream to obtainthe liquid product stream.

This embodiment according to the present invention including a flashvessel will now be discussed with reference to FIG. 3. The parts thatwere already discussed with reference to FIG. 1 have got the samereference numerals.

The column 10′ comprises an upper part 10 u and a lower part 10 l,wherein the upper part performs the function of the column 10 in FIG. 1and the lower part 10 l is a flash vessel operating at a pressure thatis below the pressure in the upper part 10 u. Suitably the pressure inthe upper part 10 u is in the range of from 0.2 to 0.5 MPa and thepressure in the flash vessel 10 l in the range of from 0.1 to 0.2 MPa.The person skilled in the art will readily understand that the flashvessel 10 l may be a component that is physically separated from thecolumn 10 l (i.e. at a certain distance).

During normal operation, liquid from the two-phase fluid suppliedthrough conduit 9 is collected in the bottom 16′ of the upper part 10 uof the column 10′. From that bottom 16′ is withdrawn a liquid streamhaving a reduced content of components having low boiling points throughconduit 17′. This stream is then introduced into the flash vessel 10 lat a low pressure. The pressure reduction is achieved by means ofJoule-Thompson valve 40 in conduit 17′. Consequently a two-phase mixtureis formed and that is introduced via inlet device 41 into the flashvessel 10 l.

Through conduit 17″ a liquid stream having a reduced content ofcomponents having low boiling points is removed, which is passed to thestorage tank 20.

From the top 23″ of the flash vessel 10 l a second gaseous stream isremoved.

Suitably the second gaseous stream is passed through conduit 42 to heatexchanger 27, in which the second gaseous stream is heated by heatexchange with the recycle stream supplied through conduit 34 a. Theheated stream is compressed in compressor 45, the heat of compression isremoved in heat exchanger 48 and passed through conduit 49 to add thecompressed second gaseous stream to the recycle stream in conduit 34 a.

It will be understood that compressors 45 and 30 can be combined intoone compressor (not shown). In that case, conduit 42 is connected to thesuction end of that compressor, conduit 28 to an intermediate inlet andconduit 32 is connected to the discharge end of that compressor.

An advantage of this method is that it can be used for largeliquefaction plants.

As with the embodiment discussed with reference to FIG. 1, theembodiment discussed with reference to FIG. 3 can as well be providedwith an auxiliary compressor to compress the recycle stream separatedfrom the fuel gas to an elevated pressure before it is at least partlycondensed in heat exchanger 27. The high-pressure recycle stream can beused in several ways, which will be discussed with reference to FIG. 4.The parts that were already discussed with reference to FIG. 3 have gotthe same reference numerals.

The auxiliary compressor included in conduit 34 a is referred to withreference numeral 35. The auxiliary compressor 35 can be provided with acooler (not shown) to remove the heat of compression for the compressedrecycle stream. The compressed recycle stream is partially condensed bycooling it in heat exchanger 27. Part of the cold that is needed isprovided by the gaseous stream that is enriched in components having lowboiling points that is passed through conduit 25. The remainder isprovided by the recycle stream. Cold from the recycle stream can beobtained by expanding a part of the recycle stream to an intermediatepressure in Joule-Thompson valve 38, using the expanded fluid to coolthe recycle stream in conduit 34 a and supplying the expanded fluidthrough conduit 38 a to the compressor 30. The intermediate pressure towhich the part of the recycle stream is expanded is in the range of fromthe suction pressure to the discharge pressure of the compressor 30(ends of the range included). The point at which the expanded recyclestream enters the compressor 30 is so selected that the pressure of theexpanded recycle stream matches the pressure of the fluid in thecompressor 30 at the inlet point.

The remainder of the recycle stream is expanded by the Joule-Thompsonvalve 37 and introduced as reflux in the column 10 as discussed withreference to FIG. 1.

An advantage of this embodiment is that the recycle stream is expandedfrom a larger pressure and thus cooled to a lower temperature. Thisallows a warmer feed stream, for example a feed stream at −142° C.,compared to a feed stream temperature of −145° C. (in the aboveexample). Thus the temperature of the liquefied gas from the maincryogenic heat exchanger can be higher and therefore, for the sameamount of energy, more gas can be liquefied.

The elevated pressure of the fluid discharged from the auxiliarycompressor 35 is so selected that the costs of the power required todrive the auxiliary compressor 35 are less than the value of theincreased amount of gas that is liquefied.

In the above we discussed an embodiment in which the expansion is donein the expansion valves 37 and 38. However, it will be understood thatthe expansion of the recycle stream can be done in two stages, at firstin an expansion device, such as expander 36 and subsequently in theJoule-Thompson valves 37 and 38.

FIG. 4 also shows that the boil-off gas from the storage tank 20 isprovided via the conduit 22 to the suction end of the compressor 45.

It will be understood that compressors 45 and 30 can be combined intoone compressor (not shown). In that case, conduit 42 (into which conduit22 opens) is connected to the suction end of that compressor, conduit 28to an intermediate inlet and conduit 32 is connected to the dischargeend of that compressor.

Instead of supplying the expanded fluid through conduit 38 a to thecompressor 30, the expanded fluid can be supplied to an inlet (notshown) of the compressor 35.

An alternative of the embodiment shown in FIG. 4 is shown in FIG. 5,wherein a part of the recycle stream that is passed through conduit 34 ais separated therefrom and passed through conduit 50 through the heatexchanger 27. Then the cooled recycle stream is expanded to theintermediate pressure in expander 51 and used to cool the recycle streamin conduit 34 a. The expanded stream is then introduced into thecompressor 30 at an intermediate stage.

Suitably, the recycle stream passed through conduit 34 a is between 10and 90% by mass of the fuel gas that is passed through conduit 31.

FIG. 6 shows the process according to FIG. 4 wherein the column 10 ucomprises two contacting sections 14. The skilled person will readilyunderstand that more than two contacting sections 14 may be present.

From between the contacting sections 14, a stream is removed via drawoff device 63 and fed via line 60 to a heat exchanger 61, wherein thestream is heat exchanged against the stream in line 1. Subsequently thestream in line 60 is returned to the column 10 u and fed via vane inletdevice 62.

In the embodiments discussed with reference to the Figures, thecontacting section 14 contains trays, however, any other contactingmeans such as packing can be employed as well. The length of the packedsection is then preferably equivalent to between two and eightcontacting trays for the section above vane inlet device 12 and betweenfive and fifteen trays for the section below draw-off device 63.

The method of the present invention provides a simple way of reducingthe amount of components having low boiling points in a liquefiednatural gas stream.

1. A method of treating liquefied natural gas supplied at liquefactionpressure containing components having low boiling points to obtain aliquid product stream having a reduced content of components having lowboiling points, which method comprises the steps of: (a) allowing theliquefied gas to expand to separation pressure to obtain an expandedtwo-phase fluid; (b) introducing the expanded two-phase fluid into acolumn below a gas-liquid contacting section arranged in the column; (c)collecting in the bottom of the column liquid from the two-phase fluidand withdrawing from the bottom of the column a liquid stream having areduced content of components having low boiling points; introducing theliquid stream into a flash vessel at a low pressure; removing a secondgaseous stream from the top of the flash vessel; and removing from thebottom of the flash vessel a liquid stream to obtain the liquid productstream; (d) allowing vapor from the two-phase fluid to flow through thecontacting section; (e) withdrawing from the top of the column a gaseousstream that is enriched in components having low boiling points; (f)heating the gaseous stream obtained in step (e) in a heat exchanger toobtain a heated gaseous stream; (g) compressing the heated gaseousstream obtained in step (f) to fuel gas pressure to obtain fuel gas; (h)separating a recycle stream from the fuel gas obtained in step (g); (i)at least partly condensing the recycle stream obtained in step (h) toobtain a reflux stream; and (j) introducing the reflux stream obtainedin step (i) at separation pressure into the column above the contactingsection.
 2. The method according to claim 1, further comprising heatingthe second gaseous stream in the heat exchanger; compressing the secondgaseous stream to fuel gas pressure; and adding the second gaseousstream to the recycle stream.
 3. The method according to claim 1,wherein at least partly condensing the recycle stream comprisesindirectly heat exchanging the recycle stream with the gaseous stream(s)in the heat exchanger.
 4. The method according to claim 1, whereincompressing the heated gaseous stream to fuel gas pressure to obtainfuel gas further includes removing the heat of compression.
 5. Themethod according to claim 1, wherein the recycle stream separated fromthe fuel gas is compressed to an elevated pressure before it is at leastpartly condensed.
 6. The method according to claim 2, wherein at leastpartly condensing the recycle stream comprises indirectly heatexchanging the recycle stream with the gaseous stream(s) in the heatexchanger.
 7. The method according to claim 2, wherein compressing theheated gaseous stream to fuel gas pressure to obtain fuel gas furtherincludes removing the heat of compression.
 8. The method according toclaim 3, wherein compressing the heated gaseous stream to fuel gaspressure to obtain fuel gas further includes removing the heat ofcompression.
 9. The method according to claim 2, wherein the recyclestream separated from the fuel gas is compressed to an elevated pressurebefore it is at least partly condensed.
 10. The method according toclaim 3, wherein the recycle stream separated from the fuel gas iscompressed to an elevated pressure before it is at least partlycondensed.
 11. The method according to claim 4, wherein the recyclestream separated from the fuel gas is compressed to an elevated pressurebefore it is at least partly condensed.